Distortion
measure-
ment
Demand
calculation
BLOCK
ALARM
U_A_AB
U_B_BC
U_C_CA
Figure 358: Functional module diagram
Distortion measurement
The distortion measurement module measures harmonics up to the 11th harmonic.
The total harmonic distortion THD for voltage is calculated from the measured
harmonic components with the formula
(Equation 76)
U
k
k
th
harmonic component
U
1
the voltage fundamental component amplitude
Demand calculation
The demand value for THD is calculated separately for each phase. If any of the
calculated demand THD values is above the set alarm limit
THD alarm limit
, the
ALARM output is activated.
The demand calculation window is set with the
Demand interval
setting. It has
seven window lengths from "1 minute" to "180 minutes". The window type can
be set with the
Demand window
setting. The available options are "Sliding" and
"Non-sliding".
The activation of the BLOCK input blocks the ALARM output.
10.2.5 Application
VMHAI provides a method for monitoring the power quality by means of the voltage
waveform distortion. VMHAI provides a short-term three-second average and long-
term demand for THD.
10.2.6 Signals
1MRS758755 C
Power quality measurement functions
REC615 & RER615
Technical Manual
705
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